Most bicycles have multiple gears with front and rear derailleurs to change gears. A front derailleur is used to shift a bicycle chain between two or more chain rings of a crankset. Chain rings vary in diameter, each having a different number of teeth for engagement with the bicycle chain. The force required to turn the crankset is determined, in part, by the size of the particular chain ring the bicycle chain is engaged with. The size of the chain ring can also determine the number of revolutions that will be required of the crankset in order to travel a certain distance. By changing the chain engagement from a chain ring of one size to another, the bicyclist can strategically choose how much force will be required to be applied to the crankset, as well has how many revolutions the crankset will make in order to travel a relative distance.
Conventional bicycle frames are configured with a seat tube joined at one end to a down tube with a bottom bracket lug. The bottom bracket lug supports the “bottom bracket” consisting of the bottom bracket spindle, bearings, races, lock rings, spacers and the like, that are configured to couple with, and allow rotation of, the crankset. The opposite end of the seat tube conventionally supports a seat post and seat for the rider. A front derailleur is typically mounted on the seat tube of a bicycle frame and near the chain rings. Not all bicycle frames have a complete seat tube. For example, some full-suspension mountain bikes have a short seat tube stub for mounting a front derailleur that serves the purpose of a frame structural member for mounting the front derailleur. For simplicity of discussion, it will be assumed that the term “seat tube” denotes a full conventional seat tube or the more recent short seat tube stub for mounting the front derailleur.
A crankset attaches to opposite ends of a bottom bracket spindle. One arm of the crankset is simply a crank arm with a pedal designed to engage the bottom bracket spindle. The other arm of the crankset may include up to 3 chain rings. A front derailleur is required to shift the chain between the chain rings. The chain rings are typically arranged so as to be concentric with, and parallel to, each other. The smallest chain ring is typically closest to the bicycle frame with the chain rings progressively growing in diameter as they get further from the bike frame. The purpose of a conventional front derailleur is to urge the bicycle chain laterally away from the currently engaged chain ring and towards a chain ring selected by the rider through means of a properly connected shifting device also known as a “shifter.” Front derailleurs of various designs have been utilized to perform this function and most have similar features.
Construction and operation of a basic front derailleur are described in U.S. Pat. No. 4,734,083 to Nagano. The Nagano front derailleur includes a member for fixing the front derailleur to the bicycle, a four pin linkage mechanism to effect a pantographic type movement of two guide members, the guide members typically being designated as the inner guide member and the outer guide member. When the assembly is activated to move away from the bicycle frame and toward a larger chain ring, the inner guide pushes the bicycle chain laterally away from the bike frame until the bicycle chain leaves the currently engaged chain ring and engages the next chain ring. The action of urging the bicycle chain onto a larger chain ring often involves the step of pressing the bicycle chain against the side of the larger chain ring. By pressing the bicycle chain against the chain ring, friction combines with the circular motion of the chain ring to “pull” the bicycle chain up and onto the chain ring. When the assembly is activated to move toward the bicycle frame and toward a smaller chain ring, the outer guide urges the bicycle chain in a similar manner, with the outer guide pressing the bicycle chain off of the larger chain ring, and then allowing the bicycle chain to fall onto the smaller chain ring.
The inner and outer guides are spaced at a width which is wider than that of the bicycle chain, often one and a half to two times as wide as the bicycle chain. This allows the bicycle chain to run between the guides without contacting the guides while also allowing for some lateral movement of the bicycle chain which occurs when the bicycle chain is changed from one gear to another by a rear derailleur on a rear sprocket set also referred to as a “cluster” or “cassette”. The inner and outer guides each typically comprise a substantially flat elongated surface. The guides are placed substantially parallel to one another and are connected together with a front and rear link. An elongated spatial channel, often referred to as a cage is, thus, defined by the inner and outer guides and the front and rear links. The bicycle chain then passes through the cage without touching the inner and outer guides or the front and rear links during normal operation. There are typically multiple links of chain within the spatial area defined by the cage at any given time.
Conventional front derailleurs move a conventional chain guide in an essentially lateral movement away from or toward the bicycle frame when shifting. There is very little vertical movement associated with conventional front derailleur shifting. For example, a conventional mountain bike front derailleur, such as the Shimano® XTR™ model FD-952 front derailleur, provides approximately 2.1 cm of lateral (horizontal and essentially perpendicular to any plane of any chain ring) movement of the chain guide when shifting from smallest to largest chain ring or vice versa over a triple crankset. As the movement of such a conventional front derailleur is not precisely linear, there is approximately 1-2 mm of vertical movement during a full sweep of the chain guide movement. Similarly, a conventional road bike front derailleur, such as the Campagnolo® Super Record™ front derailleur, popular in the early 1980s for road racing bicycles, provides approximately 1.0 cm of lateral movement of the chain guide when shifting from smallest to largest chain ring or vice versa over a double crankset. Again, this conventional front derailleur is not precisely linear. So, it exhibits approximately 1-2 mm of vertical movement during a full sweep of the chain guide movement.
Thus, there exists a need in the art for a new front derailleur with a chain guide configured to lift a corner of the bicycle chain (as viewed in cross-section) to initiate an up-shift and that pulls down on an opposite corner to initiate a down-shift. Such a chain guide and front derailleur may have improved shifting performance relative to a conventional front derailleur and chain guide.